The use of pressurized gas to assist in a conventional plastic injection molding process is believed to have been first made commercially practicable by the invention of Friederich disclosed in U.S. Pat. No. 4,101,617, issued July 18, 1978. The Friederich patent addressed the problem of molding hollow shaped bodies in a single injection molding operation, and taught a practicable method of introducing compressed gas along with, or just after, the injection of molten plastic resin into the article-defining cavity. Moreover, the Friederich patent solved the concern of de-pressurizing or relieving the molded article by nozzle separation. The early work of Friederich was directed to the molding of such utilitarian articles as clear plastic architectural bricks and the like. More recently, the patented Friederich process has been adapted to the molding of hollow plastic articles of various shapes and dimensions.
In its early years, the use of pressurized gas in assistance to a conventional plastic injection mold process was not recognized for all of the functional attributes which it is known to enjoy today. More specifically, during those early years, the industry gave greater focus to the use of structural foam as a specialty process for molding relatively thick-sectioned articles which would be light in weight and have acceptable surface finish, i.e., avoid sink marks associated with the conventional plastic injection molding. The range of potential applications of structural foam molding of thermoplastic materials was limited, however, due to certain inherent features of such process. Among such features included, the relatively long cycle times required to cool the plastic in the mold (the foam cells serve to insulate heat transfer), and the problem of surface finish (splay, blister and swirl) associated with the foamed, molten plastic resin contacting the cool surface walls of the article-defining cavity.
In recent years, attention has returned to the use of gas assistance with conventional plastic injection molding to attain the product quality and productivity which had been hoped for with structural foam molding. The features of surface quality, lower clamp tonnage, rapid cycle times, weight reduction, material saving and minimization of part distortion or warpage can all be obtained with proper utilization of gas assistance with a conventional plastic injection molding process. The paper titled "GAS-ASSISTED INJECTION MOLDING --THE NEW THERMOPLASTIC MOLDING TECHNOLOGY FOR EXTERIOR BODY PANELS" by Dr. Ken C. Rusch, presented at the 1989 meeting of the Society of Automotive Engineers on March 2, 1989, discusses in greater detail the relevant history of the use of gas-assistance in connection with plastic injection molding.
The impetus for the present invention was the inventor's assignment to realize the successful plastic injection molding of an automobile headlamp cover. This type of cover pivots between an open position to reveal the headlamp lens, and a closed position to conceal the headlamp lens to enhance the design aesthetics and aerodynamic profile of the automobile. The structure of the headlamp cover comprises a substantially planar panel which is intended to appear flush and continuous with the front end body panels when the cover is in the closed position. The planar section is mounted by a pair of flange arms integrally joined at the opposed ends. Each flange arm extends perpendicularly to the substantially planar cover portion.
The inventor's objective was to realize the headlamp cover as a plastic molded part to replace a conventional die-cast metal part. The die-cast metal part is relatively heavier and requires a higher rated motor or actuator to move the headlamp cover between its open and closed positions. Also, the die-cast cover has thermal cycling problems, and cannot withstand passage through the factory paint ovens where temperatures may reach or exceed 400.degree. F. without experiencing permanent deformation. The die-cast metal part thus has the further drawback of requiring installation in a secondary, off-line operation which added to the cost and complexity of automated automobile assembly.
There were several practical problems facing the inventor in realizing the headlamp cover design in a plastic molded piece. First, the exterior surface of the substantially planar portion of the headlamp cover had to be "Class A" quality and be paintable in the automated line. Any surface degradation due to sink marks, blemishes or other imperfections were unacceptable for commercial standards. Second, the headlamp cover could not have warpage or distortion across its substantially planar surface no matter the microscopic smoothness of the surface. Third, the dimensional integrity of the flange arms was critical to ensure safe and consistent mechanical operation of the headlamp cover when actuated between its closed and open positions. Fourth, the part had to be filled out, i.e., the part design had to be such that it could be filled out on a regular production basis within processing temperatures which do not cause degradation of the resin composition and at injection pressures which do not create molecular shear forces in the resin which would reduce the mechanical strength of the finished article. The part had been exhaustively molded in conventional plastic injection molding processes on a prototype development basis without desired success.
The dilemma facing the inventor can be simply put as follows: (i) too little plastic injection pressure would risk inferior surface quality and non-fillout of the part; (ii) too high of an injection pressure and/or injection speed would risk molecular shearing of the material and cause molded-in stress which is manifested by distortion and lack of strength; and (iii) too great of a resin temperature would decrease the viscosity of the molten resin but risk exceeding the material processing temperature and cause material degradation and dehomogenization of any filler material, such as impregnated glass.
The solution to the inventor's dilemma proved to be the use of gas-assistance in plastic injection molding the headlamp cover. However, the design did not admit to having the gas enter the article-defining cavity and form hollow portions or channels in the finished part. The substantially planar portion of the headlamp cover was required to be of continuous cross-sectional thickness, and the presence of gas channels, as well as the possibility of gas permeation outside of the channels, was unacceptable as a matter of customer dictated part design. Moreover, after a number of attempts to design the headlamp cover part with gas channels in selected positions, it was found that the mechanical strength of the cover may be compromised if gas channels were present, especially at the integral lines of joinder of the flange arms to the cover panel. The inventor's solution then turned to the novel type of gas assisted plastic injection molding process which is summarized presently.